Transducers, such as those employed in the medical ultrasound imaging industry, typically employ piezoelectric ceramic materials such as lead-zirconate-titanate (PZT) to both emit and receive ultrasound waves. In recent years, however, composites have become the focus of piezoelectric research and development. Composite piezoelectric materials, typically combinations of polymers and ceramics, can increase the desired properties of the transducer while virtually eliminating the inherent weaknesses of the individual materials. A ceramic-polymer piezoelectric composite for example, retains a high piezoelectric coefficient and moderate dielectric loss which is characteristic of a typical piezoelectric ceramic, while decreasing the overall density and brittleness of the transducer. Moreover, polymers allow for flexibility and shape variability in the final composite. As a result, the properties of such composites are far superior to those of bulk ceramics or piezoelectric polymers alone.
Composite piezoelectric transducers, and methods for their production, are described, for example, in Composite Piezoelectric Transducers; R. E. Newnham et al; Materials in Engineering, Vol. 2, December 1980, 93-106, which is incorporated herein by reference.
Piezoelectric composites typically consist of a polarizable phase embedded in a non-polarizable material. These composites have many advantages over traditional monolithic piezoelectric ceramics including: (i) lower densities resulting in acoustic impedances closer to those of the human body, water, etc., eliminating the need for an acoustic matching layer, (ii) low dielectric constants, resulting in high piezoelectric voltage constant g, and (iii) ease of conformability to the shape of the backing material of the composite.
Two composite designs that have been particularly successful are composites with 2--2 and 1-3 connectivity. In these designs, the piezoelectric or polarizable phase is aligned in the poling direction of the composite (in a matrix of a suitable polymer.) In the case of the 2--2 composite, both the ceramic and polymer phases are two-dimensionally connected throughout the composite. The stiff ceramic phase supports most of the stress applied in the direction of its alignment, yielding a high piezoelectric charge coefficient d.sub.33, while the composite maintains a low density and dielectric constant.
In the 1-3 composite, the ceramic phase is one-dimensionally connected through the composite, while the polymer phase is three-dimensionally connected. For certain applications, the 1-3 composite yields superior properties to those described above for the 2--2 composite due to the lower density and lower dielectric constant. Connectivity, including types 1-3 and 2--2, are described in detail in Composite Piezoelectric Transducers; R. E. Newnham et al; Materials in Engineering, Vol. 2, December 1980, pages 93-106, which is incorporated herein by reference.
A common and convenient method for making 2--2 and 1-3 composites is to start by cutting parallel slots into a monolithic piezoelectric ceramic block. The slots are then filled with a polymer. Typically, this polymer filler is non-polarizable. If one set of parallel slots are cut, the resulting composite has a 2--2 connectivity. If a second set of parallel slots are cut orthogonally to the first set of slots and backfilled with a filler material, the resulting composite has a 1-3 connectivity. Transducer fabrication continues by grinding the unslotted section of the monolithic ceramic away from the slotted section, lapping the two sides parallel to one another, and electroding the surfaces containing exposed ceramic. The resulting structure essentially comprises a semiflexible mat consisting of strips, posts or rods of piezoelectric material laterally encased by polymeric matrix material such as epoxy.
A strong electric field is then applied to the composite in a direction perpendicular to the plane of the polarizable sheet using conventional poling or corona discharge techniques. The intensity of the electric field used will ordinarily be selected to provide efficient polarization. However, it will be kept below the range at which substantial dielectric breakdown of the material being polarized occurs. The aforementioned method is known as the "dice and fill" method and is described in PZT-Epoxy Piezoelectric Transducers: A Simplified Fabrication Procedures, H. P. Savakus et al; Materials Research Bulletin, Vol. 16, 1981, pages 677-680, which is incorporated herein by reference.
Although, as mentioned above, piezoelectric composites typically consist of a polarizable phase embedded in an non-polarizable phase, there is a need to develop efficient manufacturing method for production of composites having multiple polarizable and/or non-polarizable phases. Also, there is a need to develop more efficient methods for the manufacture of piezoelectric composites having decreased size and/or periodicity of the polarizable phase or phases. Composites having these properties have been identified as a key area of transducer development.
Some examples of these needs are evidenced in the field of medical imaging. The resolution of imaging systems can be enhanced by increasing the frequency of the generated sound wave from the transducer. To increase the frequency of the sound wave, the size and periodicity of the polarizable phase in the composite must be decreased. In addition, enhanced pulse-echo properties are realized if the transducer contains transmitting and receiving sections wherein the transmitting sections contain an polarizable phase with high transmitting sensitivity, while the receiving sections contain a polarizable phase with high receiving sensitivity.
Moreover, there has been a drive to create so-called "smart" materials. Smart materials are described in "Smart Ceramics"; Newnham et al., Ferroelectrics, Vol. 102, pp. 259-266 which is incorporated herein by reference. A smart material senses a change in the environment, and using a feedback system, makes a useful response. It may be both a sensor and an actuator. A very smart material can tune its sensor and actuator functions in time and space to optimize behavior. Tuning of a very smart material can be accomplished by using a multitude of polarizable phases.
Finally, it would be desirable to have an efficient method to tailor the piezoelectric properties within a single transducer by varying the ceramic volume content across the device. This reduces the out of plane distortions of the transmitted signal.
The "dice and fill" method has several limitations in meeting the above stated needs. Dicing technology utilizes thin, diamond coated blades rotating at high speeds. The blades, often thinner than 200 microns, are expensive, difficult to handle, and have short usage lives. Moreover, the "dice and fill" technique uses monolithic ceramics, ruling out the possibility of composites with multiple polarizable ceramic phases. Finally, varying the ceramic volume content across the transducer would require frequent changes of dicing blades to vary the thickness of the slots, substantially adding to the time it takes to produce a transducer.